Welding Tips and Tricks

how do you guys back purge flats or inside corners?

a part came into work the other day and i noticed the welded the outside corner and the back of it (inside corner) was all sugar. no way in to grind that off.
but on a different part (different company) they welded it in the inside corner then ground the sugar off the outside.

any thoughts?

What kind of parts, is it some thing that can be done with a purgebox and alfoil type stuff?

Just thinking too, not everyone has a purge box lying around. I didnt, had to make one, An aluminium rhs or flats welded together, etc in whatever shape you need to make and just drill holes in it where you need them. A steel rhs would work too but keeping it from contaminating the job would be the only issue. A bit ss as a spacer? And just put a gas nipple or heck anything you can tape gas hose too. You would only need 5lpm or so.

one was a deep tray, the other was a bracket.
i often use a aluminium chill block which works ok IF you get a nice tight fit, but thats the hard part.

I’ve made myself several purge jigs from aluminum angle and flat stock for inside and outside corners. Essentially for the inside, I milled the ally miter down until I had about a 0.090 slot to release argon. Then I thread a quick connect into the part. Attach and detach a separate argon hose so you can set your flow and not chase back and forth to the bottle so you avoid wasting purge gas.

Has worked great. The outside jig is simply a reverse orientation of the interior. Cost me about $5 in scrap material I had laying around after other jobs. Have done the same with round stock billets for tubing. Turned down several “steps” in some round stock. Drilled a hole, thread a quick connect. Done. Fits a wider range of tubing.

I've never personally done stainless purging but I do have to make a 10'x10' stainless test tank for the shop soon which has me thinking about it.


So my beat ideas are as CJ said a ally angle with it's hard 90 edge milled down to allow argon flow.

For flat but joints my beat idea so far is either a piece of flat bar with a slot/groove milled into it. Or a 'U' extrusion with the ends welded shut to keep the argon from going out one end rather than the length of the joint.

Think the flat bar might be better option since it'll double as a chill bar vs a 1/8 U


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Toggatug wrote: ↑Mon Oct 25, 2021 6:53 pm I've never personally done stainless purging but I do have to make a 10'x10' stainless test tank for the shop soon which has me thinking about it.

For flat but joints my beat idea so far is either a piece of flat bar with a slot/groove milled into it. Or a 'U' extrusion with the ends welded shut to keep the argon from going out one end rather than the length of the joint.

Think the flat bar might be better option since it'll double as a chill bar vs a 1/8 U

Get a piece of ally rectangular tubing. Cap the ends, thread for a gas inlet. Drill a series of holes (1/16”, spaced apart in 3-4 columns) across the width. Clamp this to the butt joint in the backside. Double duty for a chill block and a gas field.

cj737 wrote:

Toggatug wrote: ↑Mon Oct 25, 2021 6:53 pm I've never personally done stainless purging but I do have to make a 10'x10' stainless test tank for the shop soon which has me thinking about it.

For flat but joints my beat idea so far is either a piece of flat bar with a slot/groove milled into it. Or a 'U' extrusion with the ends welded shut to keep the argon from going out one end rather than the length of the joint.

Think the flat bar might be better option since it'll double as a chill bar vs a 1/8 U

Get a piece of ally rectangular tubing. Cap the ends, thread for a gas inlet. Drill a series of holes (1/16”, spaced apart in 3-4 columns) across the width. Clamp this to the butt joint in the backside. Double duty for a chill block and a gas field.

So I'm going to guess that the SS doesn't necessarily need to be 'free floating' from the purge fixture?

I just always thought you needed a cavity of some sort for the argon to be in up against the metal being back purged.

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Toggatug wrote: ↑Mon Oct 25, 2021 10:36 pm I just always thought you needed a cavity of some sort for the argon to be in up against the metal being back purged.

For active purging with argon, yes.

But you can also prevent oxygen from reaching the back of a weld and the resulting sugaring by other means like fitting the pieces in a jig/frame that tightly covers the back without any gaps.

Eg. a butt-joint clamped onto a block of aluminium (so it won't 'stick' to the weld) will not have sugaring on the back. Similar an ouside corner joint that's clamped onto a square bar. Simply because the outside air can not get to it and he argon from the torch will fill the joint from the front. (the alu also helps as it draws a lot of the heat out of the weld quickly reducing the chance of it oxidising)

For production work it can be feasible to have jigs made (eg. CNC'ed) to fit all the parts that need to be welded tightly and then work that way. These jigs will be quite expensive to have made, but for bigger production runs they would pay for themselves in speed and ease of work.

But in many cases when you work on individual parts or joints these are shaped oddly and then you need to use various shapes of purge equipment that creates a constant flow/cover to the back of the joint and prevents the sugaring or make use of some of the adhesive tape type backings that work similarly to clamping something to the back of the joint or (if applicable) use something like SolarFlux-B to protect the back (but that will result in a glass-like deposit over the back of the weld)

It also depends a lot on what the final product will be used for. For food-grade work then usually the only acceptable method is real back purged fusion welds with full penetration. If it's for other purposes then the other methods could be applicable.

Bye, Arno.

Arno is effectively correct. Ally blocks clamped do limit the atmosphere but won’t totally expel it. That’s the reason I use the fixture that is still “bleeding” argon between the joint and clamped fixture. Air is pretty small molecule. Sure, clamped pieces help prevent sugaring, but can’t completely eliminate it. Absent of a large purge chamber, a clamped jig with argon vents is my choice. You could also intentionally gap the joint and preflow to help evacuate the argon, but the backside is still mildly at risk.

And again to his point, this is NOT food-grade sanitation welding. It’s job shop stuff that includes fixtures, counters, pieces, etc. I’ve done a few mounting plates for boats this way (Windless anchor brackets, motor mounts, etc) as well as some lucrative counter tops for commercial and residential spaces.